We are running exchange 2003 and using Native Pop3 Connector - http://www.mapilab.com/exchange/pop3_connector/
instead of Microsofts POP3 connector to download e-mail off our external e-mail addresses. Exchange does not serve as our primary e-mail server. We use Native POP3 connector because it offers smaller intervals of times between checking the inboxes.
Lately however - our exchange server has been the target of some direct and sneaky spam. masquarading as "Undeliverable"
Seeing as our exchange server doesn't (shouldn't) receive emails directly - but instead get e-mails from our offsite e-mail server via pop3, I disabled anonymous connections in the default smtp virtual server - and whitelisted our offsite e-mail server.
However - I am not receiving e-mails now. If I send an e-mail from gmail - or other e-mail server, the Offsite server receives the e-mail, the pop3 connector downloads the e-mail - as it it shortly removed from the offsite server, but the e-mail never shows up in my exchange mailbox. If I enable anonymous authentication to the default smtp virtual server, then the e-mails show up in my exchange mailbox.